1. Field of the Invention
This invention relates to compositions and articles suitable for use in thermal transfer imaging processes, and also relates to graphic articles comprising a graphic image formed using the inventive compositions and articles, and methods of making such graphic articles.
2. Related Art
Graphic articles, sometimes referred to as signage articles, are used in a wide variety of applications, both for informational and for decorative purposes. For example, retroreflective sheetings are an important medium used in manufacturing graphic articles such as license plates, road/street markings, validation stickers, and package labels. Decals are a medium used for automotive labels and decoration.
Images on graphic articles may be formed by thermal transfer of a color layer from a first substrate or carrier, usually a plastic film, to a second substrate surface, e.g., the retroreflective sheeting or a component thereof. Thermal transfer printing methods, such as hot stamp printing or thermal mass transfer printing, form an image by selective transfer of portions of the color layer from the first substrate to the second substrate. Images formed on graphic articles may be, for example, alphanumeric characters, bar codes, or graphics.
Alternatively, images may comprise preformed color layers on a decal, which are transferred by hot transfer lamination. It is known to form graphic patterns on substrates using transfer articles bearing predesignated designs. G.B. Patent No. 1,218,058 (Hurst et al.) discloses transfers with an adhesive layer applied to only those areas intended to be transferred to the substrate; U.S. Pat. Nos. 4,786,537 (Sasaki) and 4,919,994 (Incremona et al.) disclose transfer graphic articles wherein the graphic design is formed via imagewise differential properties within the transfer film itself. One problem with such approaches is that a large and varied inventory must be maintained in order to provide a variety of graphic patterns.
Formation of desired graphic images from continuous layers via thermal transfer processes is also well known. For example, thermal mass transfer articles, typically comprising a carrier, optionally a release layer, and a transferable color layer have been known for some time. The article is contacted to a desired substrate such that the color layer is in contact with the substrate and heat is applied in imagewise fashion to cause imagewise portions of the color layer to release from the carrier and adhere to the substrate.
Hot stamping foils comprising a carrier, one or more color layers, and an adherence layer have also been known for some time. Such films have been used to provide imagewise graphic patterns, e.g., alphanumeric or decorative legends, to substrates via imagewise application of heat and/or contact or pressure. In some embodiments, additional members such as release layers are used to facilitate desired performance. In some embodiments, so-called xe2x80x9ctexture layersxe2x80x9d and/or xe2x80x9cticksxe2x80x9d, metal layers, etc. are used as well to yield desired appearance. Hot stamping foils are also sometimes called hot-stamp tapes or thermal transfer tapes.
The color layer(s), adherence layer, and any other layers that are selectively applied to the substrate should split or fracture in desired manner in order for the applied graphic pattern to have a desired edge appearance. Some illustrative examples of previously known hot stamping foils are disclosed in U.S. Pat. Nos. 3,770,479 (Dunning) 3,953,635 (Dunning), and 4,084,032 (Pasersky). It has also been known to transfer graphic patterns using means in addition to or other than heat to achieve imagewise separation of imaging material from a carrier and adhesion to a substrate. For example, U.S. Pat. No. 3,834,925 (Matsumura et al.) discloses a transfer material that utilizes solvent action to achieve imagewise deposition.
An advantage of the foregoing techniques is that the transfer film may be made as a uniform sheet, i.e., with no specific latent image embodied therein. The applicator defines the graphic pattern by controlling the application process, e.g., imagewise application of heat and/or contact or pressure permits maintenance of a smaller inventory of thermal transfer element material.
One well known use of hot stamping foils is to print legends on vehicle identification plates. For example, license plates produced using hot stamping foils have been used in Austria, Australia, Finland, Germany, Ireland, Portugal, and Switzerland. One commercially available hot stamping foil currently used on license plates with polyvinyl chloride cover films is believed to comprise a polyester carrier, about 28 microns thick; a color layer based on acrylic resins such as polymethyl methacrylate and containing carbon black pigments, about 5 microns thick; and an acrylate-based adherence layer, about 5 microns thick. Examples of resins that are believed to have been used in adherence layers include polyvinyl alcohol copolymers, nitrocellulose, and methyl methacrylate/butyl methacrylate copolymers.
Recently improved retroreflective sheetings have been made available which have cover films made of olefin-based materials or polyurethane-based materials to improve certain performance. As disclosed in the aforementioned U.S. Pat. No. 4,896,943 (Tolliver et al.), olefin-based cover films, e.g., ethylene/acrylic acid copolymers, can provide superior properties including abrasion and dirt resistance. Many conventional hot stamping foils do not achieve good adherence to such cover sheets, however, resulting in graphic patterns having unsatisfactory durability and performance.
More recently, U.S. Pat. No. 5,393,950 (Caspari) discloses hot stamping foils well suited for use on retroreflective articles wherein the foils comprise a carrier, optionally a release control layer, a color layer, and an adherence layer wherein the adherence layer comprises, and may consist essentially of, a mixture of an ethylene copolymer dispersion and an acrylic dispersion.
Graphic articles having images formed by thermal transfer normally provide satisfactory print quality, legibility, and adhesion. However, many presently known thermal transfer color layer formulations are compatible with only a limited class of retroreflective sheeting layers, primarily those layers comprising polyvinyl chloride (PVC), acrylics and polyurethanes. Sheetings having polyvinyl butyral, ethylene/acrylic acid copolymer, or melamine/alkyd copolymer surface layers may be difficult to print upon by known thermal transfer methods. Furthermore, PVC is not environmentally desirable.
Retroreflective articles are typically provided with cover films (e.g., made of polymethylmethacrylate (PMMA), plasticized PVC, alkyd resins, acrylic resins, and the like) to improve retroreflective performance under wet conditions and to protect the retroreflective elements.
To provide improved durability, embossability, and abrasion resistance, improved retroreflective sheetings with substantially thermoplastic cover films were developed, for example aliphatic polyurethanes and ethylene/acrylic acid copolymers, that latter including ionomers. One of the problems with the newer substantially thermoplastic cover film materials is that in order to achieve satisfactory adhesion of thermal transfer color layers to such films, chemical and/or physical priming of the cover film may be needed. For example, U.S. Pat. No. 5,393,590 (Caspari) discloses a hot stamp foil having a novel adherence layer over the color layer that permits effective thermal transfer printing upon polyolefin- or polyurethane-based surfaces.
Color layers of hot stamp foils and thermal transfer ribbons generally have low cohesive strength, which promotes efficient and desired imagewise transfer of color layer material from the first substrate to the second substrate surface during thermal transfer. However, images formed from such color layers should exhibit sufficient durability for many end uses. To improve the durability of the image, a protective layer often is formed thereover, e.g., by incorporating a cover layer in a hot stamp foil to be transferred along with the color layer, or by applying a clear coat over the transferred color layer.
U.S. Pat. No. 5,468,532 (Ho et al.), and a continuation-in-part thereof, U.S. patent application Ser. No. 08/506,926, filed Jul. 26, 1995, (Phillips), now pending, both incorporated herein by reference, disclose multilayer graphic articles comprising a substrate, a color layer disposed on the substrate, and a transparent, protective layer that overlies both the color layer and the substrate. The color layer is formed from ink formulations comprising a color agent and a copolymeric binder formed from the copolymerization product of an olefinic monomer and a second monomer having a pendant carboxyl group. The binder may be crosslinked through an ionic bond, a covalent bond, etc. In each case where the binder is crosslinked, carboxylic acid groups pendant from the copolymeric binder participate in the crosslinking reaction.
Although the above work is impressive, there still exists a need for thermoplastic thermal transfer compositions and thermal transfer articles that allow the thermoplastic composition to be readily and easily applied to a variety of substrates without using chemical and/or physical priming of the substrate, and which produce durable, weatherable images. Durability and weatherability of the compositions after being transferred to the substrate, with or without a cover layer, is also a primary goal.
In accordance with the present invention, thermal transfer compositions and articles are presented which overcome many of the problems encountered with known compositions and articles. Thermal transfer articles of the invention comprise a carrier, optionally a release layer, a color layer releasably adhered thereto, and optionally an adherence layer on the bottom side of said color layer. Articles of the invention can be in the form of mass thermal transfer ribbons or hot stamp foils. The invention also provides novel graphic articles and methods for making same.
Unlike previously known thermal transfer articles, the articles of the invention 1) exhibit thermoplastic, low cohesive properties during transfer such that good image resolution and transfer is achieved; and 2) are subsequently radiation crosslinked such that a durable image is formed. The novel combination of thermal transfer of a thermoplastic material and subsequent radiation crosslinking disclosed herein is unobvious and provides previously unattained performance advantages.
The invention also provides a thermal transfer imaging process using thermal transfer elements, sometimes referred to as donor elements, of the invention. The invention also provides graphic articles bearing images formed via the method of the invention. In accordance with the process and materials of the invention convenient generation of desired images that are durable is readily attained.
One aspect of the invention is a coatable, radiation-crosslinkable thermoplastic composition. An illustrative example of such a composition is:
a) a dispersion of copolymer with the general formula: 
wherein R1 is selected from the group consisting of H and alkyl groups containing one to eight carbon atoms; R2 is selected from the group consisting of H, alkyl groups containing one to six carbon atoms, xe2x80x94CN, ester groups and R3xe2x80x94COOH, wherein R3 is any alkyl group, e.g., containing one to eight carbon atoms; X and Y are independently selected from the group consisting of a residue of the first ethylenically unsaturated monomer and a residue of the second ethylenically unsaturated monomer; n is a positive integer selected such that the first ethylenically unsaturated monomer provides from about 70 to 99 mole percent (48 to 97 weight percent) of the copolymer; and m is a positive integer selected such that the second ethylenically unsaturated monomer correspondingly provides from about 1 to 30 mole-percent (2 to 52 weight percent) of the copolymer (which is sometimes referred to hereinafter as Component A);
b) a crosslinkable polyurethane comprising a plurality of pendant ethylenically unsaturated moieties (the polyurethane is sometimes referred to hereinafter as Component B); and
c) a colorant. Various combinations of such materials may be used to advantageous effect.
Compositions of the invention typically include a photoinitiator, e.g., an ultraviolet light photoinitiator. The copolymer optionally comprises an ethylene copolymer, more preferably ethylene/acrylic acid copolymer.
Preferably, the composition is suitable for forming a covalently crosslinked, substantially thermoset, durable and weatherable (as defined herein) graphic image on a substrate surface. The substrate may comprise, for example, polyvinyl butyral, melamine alkyd/urethane, polymethylmethacrylate, polyvinyl chloride, polyurethane and ethylene copolymers (such as extruded ethylene/acrylic acid copolymers, ionomers, and the like). Materials such as ethylene/acrylic acid copolymers are typically difficult to durably adhere to.
As used herein the terms durable and durability refer to characteristics such as solvent and chemical resistance, abrasion resistance, bond maintenance of the solid residue of the inventive composition to the substrate, and maintenance of color brightness (and for retroreflective substrates, retroreflective brightness). The terms weatherable and weatherability refer to the characteristics such as maintenance of retroreflective brightness, resistance to dirt, resistance to yellowing and the like, all of these in normal use conditions in the outdoors, where sunlight, temperature, and other environmental parameters may affect performance.
Crosslinking is typically activated by actinic irradiation of the photoinitiator after the graphic image is formed, preferably with ultraviolet light. Useful actinic radiation typically has an energy dosage ranging from about 150 to about 400 milliJoules/centimeter2 of substrate surface. The crosslinking can be also be induced by exposing the material to be crosslinked to an electron beam. As used herein, the term radiation refers to any form of energy that causes the crosslinking reaction, including actinic radiation and electron beam exposure.
Another aspect of the invention is a first graphic article comprising:
a) a substrate having a polymeric surface layer referred to here as an image receiving layer; and
b) a graphic layer adhered to at least a portion of the image receiving layer, the graphic layer comprising an effective amount of a colorant and a crosslinked binder, the binder derived from the crosslinkable composition of the invention. The graphic layer may be imagewise and discontinuous, or the graphic layer may be a substantially uniform, continuous layer.
Preferred image receiving layers comprise materials previously mentioned as suitable substrate surfaces. Preferred articles of the invention are retroreflective through inclusion in the substrate of retroreflective elements. The articles may further comprise a cover film overlying the graphic image and the image receiving layer if desired.
Another aspect of the invention is a second graphic article comprising:
a) a substrate having a polymeric surface layer referred to here as an image receiving layer; and
b) a graphic layer adhered to at least a portion of the image receiving layer, the graphic layer comprising:
i) a primer layer adhered to at least a portion of the image receiving layer, the primer layer comprising a first crosslinkable binder; and
ii) a colorant layer adhered to the primer layer, the colorant layer comprising a colorant and a second covalently crosslinked binder which is at least partially covalently crosslinked with the first crosslinkable binder layer.
Another aspect of the invention is a first method of forming a graphic article, the method comprising the steps of:
a) providing a substrate having an exposed polymeric surface layer referred to here as an image receiving layer;
b) providing a thermal transfer article effective for forming a graphic image on the image receiving layer, the thermal transfer article comprising a removable carrier having first and second major surfaces and a colorant layer on one of the major surfaces, the colorant layer comprising a solid residue of a coatable, crosslinkable thermoplastic composition;
c) contacting the colorant layer to the image receiving layer of the substrate;
d) forming the graphic image by thermally transferring at least a portion of the colorant layer to the image receiving layer to yield a graphic article precursor;
e) removing the carrier from the graphic article precursor; and
f) crosslinking the graphic image (e.g., by exposure to actinic radiation).
Another aspect of the invention is a second method of forming a graphic article, the method comprising the steps of:
a) providing a substrate having a surface layer referred to here as an image receiving layer;
b) coating a primer layer onto the image receiving layer, the primer layer comprising a polymer such as an urethane, an acrylic or a blend of urethane and acrylic, with such polymer having a plurality of pendant ethylenically unsaturated moieties;
c) drying the product of step b) to remove substantially all moisture;
d) applying a radiation crosslinkable composition of the invention to the product of step c); and
e) exposing the product of step d) to radiation sufficient to crosslink the radiation-crosslinkable composition.
The primer layer may be applied in either an imagewise, discontinuous manner, or in a substantially uniform, continuous layer.
Illustrative methods within these aspects of the invention are those wherein the substrate comprises retroreflective elements, and wherein the substrate comprises an ethylene copolymer cover layer onto which the graphic image is formed.